9. Notes on Feedback Control Loops with the DigiLock 110
Page 45
Status: 5.12.17
Figure 33 Polarity of the PID controller using frequency modulation
The graph shows the characteristic distortions of the error signal as well as the saturated
absorption signal of a top of fringe lock for different polarities. The undistorted signal (solid
black) is given for reference. For the correct overall polarity (dashed red) the slope of the
error signal at the peak is shallower while the peak itself is broader (see boxed part of the
graph). In contrast for the wrong polarity (dotted green) the slope is steeper and the peak
narrower.
If the overall sign is correct (incorrect) a resonance should become wider (narrower) as compared to the
case of the controller being inactive. The slope of the error signal, e.g. in frequency modulation should
become flatter (steeper) when the overall sign is correct (incorrect).
If the overall gain is too high, oscillations will be observed, which will be either at the original position
with multiple zero crossings or repelled from the original position with oscillations positive and/or negative
at different parts of the signal, for correct and incorrect overall polarity, respectively. To suppress the
oscillations reduce the overall gain. Generally, the change in slope (as explained earlier) is easier to inter-
pret.
NOTE ! The AutoLock mode of the DigiLock 110 expects a consistent choice of the polarity setting
for all PID controllers involved, see above. Once the overall polarity is found, a correction
of the polarity can be compensated by changing the slope also.
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